1. Field of the Invention
This invention relates to a method of adjusting the recording power for recording information on an optical disk and also to an optical disk apparatus adapted to operate with such a method. More particularly, the present invention relates to a method of adjusting the recording power for an optical disk, utilizing the information written on the optical disk.
2. Description of the Related Art
To date, optimum recording power varies from optical disk to optical disk due to differences of characteristics of optical disks among manufacturers and variances of characteristics of individual optical disks. Additionally, optimum recording power depends not only on the characteristics of each optical disk but also on the timing of emission of a light beam (recording strategy), the contour of the spot of the emitted light beam and other characteristics of each optical disk apparatus. Still additionally, it is necessary to take the calibration error of recording power among optical disk apparatus into consideration.
For these reasons, an optical disk apparatus for recording data on optical disks is configured to detect the optimum recording power for each optical disk by so-called test recording in order to determine the optimum recording power of the optical disk in advance of an actual data recording operation and subsequently execute the actual writing process, using the optimum recording power.
A series of operations for detecting the optimum recording power by test recording is referred to as OPC (optimum power control).
Now, the OPC operation will be described below. Each optical disk apparatus acquires the data required for defining the optimum recording power for an optical disk from the disk information region of the optical disk arranged at the innermost track part of the disk and performs a test recording operation of writing a predetermined pattern in a recording test region.
Generally, a random pattern where marks ranging from the shortest mark to the longest mark are randomly repeated according to a modulation rule relative to the clock period T of the modulation rule is used as recording pattern for test recording. Then, the optical disk apparatus defines the recording pattern by referring to the reference recording power Ptarget that is included in the data required for defining the recording power as acquired in advance from the disk information region. In other words, the optical disk apparatus sequentially switches the recording power stepwise within a predetermined range that is centered at the reference recording power Ptarget and records the random pattern.
After recording the random pattern, sequentially switching the recording power in the above-described manner, the optical disk apparatus reproduces the random pattern and observes the signal amplitude m for each recording power. The signal amplitude m is expressed as the difference between the highest level and the lowest level of reproduction of the signal recorded to correspond to each recording power as shown in FIG. 9 of the accompanying drawings. Generally, the amplitude value m is determined by detecting the peak and the bottom of the reproduced signal amplitude.
Then, the signal amplitude m is observed for each recording power and the product of multiplication of each recording power P and the corresponding signal amplitude m, or P×m, is used as evaluation value. This method (to be referred to as κ method hereinafter) has the advantage that the accuracy level can be expectedly improved by using linear approximation.
Now, the κ method will be described below. FIG. 10 of the accompanying drawings is a typically characteristic curve, illustrating the relationship between the recording power P and the signal amplitude m. The signal amplitude m is approximated by formula (1) shown below as a function of the recording power P,m=Mmax×(1−Pthr/P)  (1),where Pthr is the smallest necessary power (recording threshold power) and Mmax is the largest value (saturation value) of the signal amplitude to be recorded. Formula (2) is obtained by multiplying the two sides by P. The relationship between the evaluation value P×m of the left side can be approximated by a straight line as shown in FIG. 11 of the accompanying drawings. Note that Pthr corresponds to the P intercept of the approximation straight line in FIG. 11.P×m=Mmax×(1−Pthr)  (2)
FIG. 11 illustrates a typical relationship between the recording power P and the evaluation value P×m. As shown in FIG. 10, the change in the signal amplitude relative to the change in the recording power is reduced as the recording power rises. In other words, the sensitivity to changes of amplitude falls. It is desirable for an OPC operation that the sensitivity is high from the viewpoint of accuracy of observation. Therefore, a predetermined factor ρ is introduced for the optimum recording power Pwopt to be used for actual recording. Thus, the recording power Ptarget for OPC is defined as Ptarget=Pwopt/ρ,
and the recording power is recommended to be used as reference for OPC. In other words, the value of Ptarget is recorded in the disk information region of the optical disk. The ratio of Ptarget and Pthr is defined asκ=Ptarget/Pthr and the value of κ is also recorded in the disk information region of the optical disk.
When Ptarget, ρ and κ are given as disk information, an OPC operation is conducted by following the following sequence.
A test recording process is executed in the OPC region (recording test region) of the disk by using a predetermined random pattern, changing the recording power at and near Ptarget within a range of Ptarget±10%, for instance. Then, the signal amplitude m corresponding to each recording power P is observed and the evaluation value P×m is computed.
Then, the relationship between P and P×m is linearly approximated and Pthr is determined from the intersection of the power axis and the approximation straight line (P intercept). The determined Pthr is multiplied by κ to determine Pt (Pt=Pthr×κ) and Pt is multiplied by ρ to determine Pwopt (Pwopt=Pt×ρ). Note that Pt is a redefinition of Ptarget by OPC. If the state of the disk and the state of the apparatus are the same as those when the value of Ptarget recorded in the disk information region of the disk was obtained, the value of Pt and the value of Ptarget should agree with each other.
A method of determining the relation of the evaluation value P×m and the recording power P within two observation ranges located near Ptarget, e.g., one centered at a point slightly shifted to the lower power side and the other centered at a point slightly shifted to the higher power side from Ptarget, and executing a predetermined computational process to improve the accuracy level of observation is also known.
With this method, Pf1 that is slightly shifted from Ptarget to the lower power side for one of the centers of power and Pf2 that is slightly shifted from Ptarget to the higher power side for the other center of power are defined. Then, also Pt1 and Pt2 are defined for the respective power centers as values obtained by determining the recording threshold powers Pthr1 and Pthr2 as the P intercepts from the relation (2) and multiplying them by a predetermined factor κ.
Then, Pf and Pt are taken respectively on the x-axis and on the y-axis and the x-coordinate (=y-coordinate) of the intersection of the straight line A connecting point (Pf1, Pt1) and point (Pf2, Pt2) and the straight line B of Pt=Pf is defined as Pt′ and Pt′ is multiplied by ρ to determine the optimum recording power Pwopt (Japanese Patent Application Laid-Open No. 2005-267802).
However, the recording power that is used for test recording by OPC can show a large discrepancy from a suitable power value depending on the characteristics of the recording film of the optical disk, the optical characteristics of the optical disk apparatus and the changes with time of the optical system such as those produced by stains.
Then, Pf1 and Pf2 of the recording powers as the power centers to be used by the apparatus for test recording can be shifted significantly from the values suited for OPC. Accordingly, the values observed at the two points of Pf1 and Pf2 can differ from the corresponding values contained in the information stored on the optical disk to a large extent.
Particularly, when the linearity of the evaluation value P×m of the optical disk is dependent on the recording power to be used for the observation to a large extent as shown in FIG. 11, the inclination of the approximation straight line can be modified significantly. Then, as a result, Pthr and Pt can be deviated greatly and the optimum recording power Pwopt computed from them can be deviated accordingly so that it may no longer be possible to correctly define the optimum recording power Pwopt.
As an example, the outcome of an experiment where an optical disk apparatus was driven to operate for test recording under a condition where the desired recording power is not obtained on the recording film of an optical disk because of the stains of the optical system will be described below by referring to FIG. 12 of the accompanying drawings. Pt1 and Pt2 in FIG. 12 are obtained as a result of the test recording.
The power center slightly shifted from Ptarget to the lower power side is defined as Pf1 and the power center slightly shifted from Ptarget to the higher power side is defined as Pf2 as in the case of the above description of the prior art. The recording threshold powers Pthr1 and Pthr2 computed from the relation (2) for the respective power centers are multiplied by a predetermined factor κ to determine Pt1 and Pt2.
Ideally, it is desirable that Pt1 and Pt2 are found at the opposite sides of the straight line expressed by Pt=Pf like Pt2 and Pt3 illustrated in FIG. 12. In such a case, the error that arises when determining Pt′ will be alleviated.
However, in FIG. 12, the power of irradiation on the optical disk is reduced so that consequently Pt1 and Pt2 that are determined respectively from Pf1 and Pf2 are deviated and moved away from the straight line of Pt=Pf representing the ideal cases. With the conventional method, the intersection of the straight line A connecting Pt1 and Pt2 and the straight line B of Pt=Pf is obtained as wanted Pt′. Then, Pt′ is multiplied by a predetermined factor ρ to determine the optimum recording power Pwopt.
However, since the range of recording power when the optical disk is operated for OPC is deviated from the desired range to a large extent, the position of point (Pf1, Pt1) and the position of point (Pf2, Pt2) are shifted greatly from the straight line Pt=Pf. Thus, it is no longer possible to properly determine Pt′ by way of the process using the two points. The net result is a deviated optimum recording power Pwopt.
Generally speaking, the recording power for recording information on an optical disk is lowered from the desired recording power level when the recording power is changed by degradation of the optical performance and stains of the optical system. Thus, there can arise an instance where nothing is recorded by the recording power for test recording as indicated by A in FIG. 11. In such an instance, the linearity of the evaluation value (P×m) gives way and hence the outcome obtained by linear approximation, using the observation points, is also deviated to a large extent.
On the other hand, when the test recording operation is repeated until the requirement of Pf1<Pt<Pf2 is met, long time is consumed for the test recording and for the reproduction of the recorded signal. Such an optical disk apparatus is inevitably a slow start apparatus that takes time until the optical disk apparatus become ready for operation.